Morphological studies of oxygen deficient hearts have been primarily limited to qualitative observations of tissue made ischemic by occlusion of the coronary artery. The model of myocardial ischemia to be used in the present study is designed to induce whole heart ischemia in isolated perfused rat hearts as a result of decreased retrograde flow through the entire coronary arterial tree. Experiments on the ischemic model will be complemented by studies on the effects of anoxia and hypoxia on the isolated perfused rat heart. The present study proposes to correlate the light and electron microscopy of aerobic and oxygen deficient hearts with their stereo ultrastructure using scanning electron microscopy. Alterations of the lysosomal system will be assessed using histochemical techniques for the demonstration of acid phosphatase. Preliminary morphometric studies indicate that early quantitative changes in the intercalated disc can be detected in ischemic hearts. In the proposed research, several parameters of cardiac ultrastructure will be studied in aerobic and oxygen deficient hearts using quantitative stereological techniques. In this way we will attempt to define the sequence of changes which take place in oxygen deficient hearts, and investigate the mechanisms by which the destruction of myocardial cells occurs during ischemia.